1. Field of the Invention
The present invention relates to a method/system for determining the touch point of a controlled vehicular master clutch, and, more particularly, relates to a method/system for tracking the value of clutch control parameter (such as actuator displacement) corresponding to clutch touch point between system calibrations (as may occur at vehicle start up) without requiring an actual system recalibration operation.
2. Description of the Prior Art
Automated vehicular master clutch systems and fully or partially automated mechanical transmission systems and the automatic controls for the master clutches thereof are known in the prior art as may be seen by reference to U.S. Pat. Nos. 3,478,851; 3,752,284; 4,019,614; 4,038,889; 4,081,065; 4,361,061; 4,401,200; 4,508,625 and 4,828,093, the disclosures of which are hereby incorporated by reference.
Briefly, in such automated systems, various drive line operations, including the engagement and disengagement of the master friction clutch, are automatically controlled by a control system, including a central processing unit, based upon certain measured, sensed, stored and/or calculated input parameters. Typically, the input parameters include engine speed, throttle position, transmission input and/or output shaft speed, vehicle speed, current engaged gear ratio, application of the brakes, condition of the clutch actuator and the like.
For automatic clutch control, during normal operation, when starting from at rest or operating at a very low speed, the master friction clutch is modulated between fully disengaged and fully engaged conditions, i.e. is partially engaged, according to certain input parameters and logic rules. In one type of automated start-from-stop system, the clutch is modulated to maintain the engine speed at a set value above idle speed and/or to achieve smooth starts. Typically, the set engine speed value is throttle position modulated to provide appropriate starting torque and the clutch is moved toward engagement and disengagement, respectively, as the engine speed increases above and falls below, respectively, the set engine speed value. In another system, see above mentioned U.S. Pat. No. 4,081,065, the clutch is modulated in accordance with throttle position, engine speed and engine acceleration.
In such automated systems, the controlled clutch is quickly moved from a fully disengaged condition to almost the incipient engaged condition (i.e. the "touch point") and then modulated to full engagement. The term "incipient engagement" of the master clutch refers to the clutch condition wherein the friction surfaces are almost or just at the point of contact.
Movement of the clutch actuators from the fully disengaged to the incipient engagement position in any manner other than the fastest possible manner is undesirable as control of the system, i.e. control of engine and/or input shaft speed, by varying amount of clutch engagement (i.e. slip) is not possible during this portion of actuator movement and delays in achieving at least incipient engagement make closed loop control of the system more difficult.
Failure to periodically monitor and update the point of incipient engagement, as determined by a monitored clutch actuator parameter, such as actuator pressure and/or a lever position, is undesirable as wear of the friction surfaces and/or mechanical actuator members, misadjustments of the mechanical members, and especially changes in clutch operating temperatures, can cause a relatively large variance in the value of the actuator parameter corresponding to incipient engagement.
Various calibration routines to determine the value of a control parameter corresponding to incipient engagement are known in the prior art, see for example U.S. Pat. No. 4,646,891, the disclosure of which is incorporated herein by reference. Such calibrations often involve shifting to a transmission neutral, possibly applying an inertia brake or input shaft brake, slowly engaging the clutch and then watching for signals assuming a value indicative of incipient/initial engagement, and thus require that the vehicle be incapacitated during the calibration. Such calibrations are often performed at vehicle start-up and/or only periodically. Between calibrations, the value of the monitored control parameter indicative of incipient engagement was assumed to remain substantially constant.
If, between calibrations, the operating temperature of the clutch changed significantly, the clutch experienced significant wear and/or the clutch operation mechanism experienced significant wear and/or deformation, then the last calibrated control parameter value assumed to correspond to incipient engagement might be significantly inaccurate resulting in degradation of the control of the controlled master clutch. In practice, as calibration operations were usually performed at least daily, changes in clutch operating temperatures accounted for much of the inaccuracies.